I tried to demonstrate to my students how boiling temperature can be used for distilling citric acid from a solution of citric acid in tap water. To my surprise, upon heating up, the colorless solution began turning yellow-brown, and a strange acrid odor filled the lab. Finally, a yellow-brown substance with a caramel-like texture remained at the bottom of the flask. I did some research and found out that citric acid is an organic compound. What exactly happened there? What are the products of this chemical reaction? How can a solution of citric acid, or any other organic compound for that matter, be separated into its components without oxidizing or changing the chemical formula?

To begin answering this question I will mention a short anecdote about Iran: Iran is one of the largest oil exporters in the world, and its land contains the third biggest oil reservoir in the world. Nevertheless, Iranians suffer from a critical gasoline shortage. The reason is that Iran has very few distilleries for distilling gasoline from crude oil, and it lacks the technological and scientific knowledge to build such distilleries. For this reason Iran is forced to import oil distillation products from other countries. This example demonstrates that distillation, particularly that of organic compounds, is no simple process, but one that requires much expertise, technical skills and secrets of the trade, that even large countries find hard to obtain.

Distillation is a technique for separating mixtures of different materials by taking advantage of their different boiling temperatures. Through a controlled process of heating, different materials can be evaporated separately from the mixture, followed by condensing them back to liquid form. Condensation is achieved by cooling down the vapors that rise from the boiling mixture. Old distillation systems contained a component called a retort – a long, curved neck through which the vapors flowed into a separate, cooled container:

An alchemist's retort

 

A modern distillation system looks like this:

The mixture to be distilled (15) is heated, the vapors rise up the still head (3) on top of which is a thermometer (4). The vapors flow through the condenser (5), which is cooled down by an external water current (6 & 7). Droplets from the condensed fluid are collected in the receiving flask (8).

 

I highlighted the thermometer, which was missing in your system. The thermometer can help us measure the temperature of the vapors that rise from the mixture, which is essential for the success of the procedure. The difference between the boiling points is at the heart of the distillation process! Lack of knowledge of the temperature is tantamount to performing the distillation blindfolded, deprived of the ability to tell when the distillation is complete. For instance, if we wish to distill alcohol from a mixture of alcohol and water (maybe to prepare whiskey from fermented malt), and we know that the boiling point of alcohol is at 78ºC (172ºF), then clearly we shouldn't heat the mixture to a higher temperature, since at this point all the alcohol has probably evaporated and the original flask only contains water. This is the point where the distillation should be stopped.

As for your experiment: the citric acid that you used was probably the food additive commonly known as "sour salt", despite it not being a salt in the scientific sense of the word (an ionic compound), but rather a pure organic compound in the crystalline form. The chemical formula of citric acid is C6H8O7, and the chemical structure is as follows:

The melting temperature of citric acid is 153ºC (307ºF), which means that it is solid at room temperature. It doesn't have a boiling point, since like many other organic compounds (like sugar for instance), it breaks down before it evaporates. At 175ºC (347ºF), citric acid breaks down completely into many small compounds, to the point of virtual carbonization (i.e. formation of a coal-like material). Citric acid is highly soluble in water. As you know, water boils at 100ºC (212ºF). Were you to include a thermometer in your system, you would have been aware of the point when all the water had evaporated (when the temperature had risen to over 100ºC) and when the heating should have been stopped. Without a thermometer, you kept heating the mixture past this point, which led to melting of the citric acid and finally to carbonization, as evident from the caramel-like material you ended up with.

To be more precise, separation of a solid from a solution is somewhat more complicated (as suggested by the Iranian anecdote – there is nothing trivial about distillation). This is because the dissolving of a solid in a liquid decreases the freezing point of the latter and elevates its boiling point. Hence, the fact that the citric acid was dissolved in water elevated the boiling point of the water, so that complete evaporation occurred at a temperature greater than 100ºC. Hence, ending the distillation process at exactly 100ºC would leave us with plenty of water in the mixture. This means that we would have to heat the mixture to a temperature far higher to get rid of all the water. However, we would then be faced with another problem: it is possible that the boiling temperature is so much higher that chemical breakdown of the citric acid may occur (complete breakdown occurs at 175ºC). This can be avoided by performing the distillation under low pressure, which would bring down the boiling point. Connecting a gas pump to the system would make the mixture boil at a lower temperature (possibly even room temperature – click here for a detailed explanation) and the water vapors would be "sucked away".

Another problem with organic compounds is their tendency to be oxidized at high temperatures in the presence of oxygen. A possible solution might be to saturate the system with a noble gas (such as argon) which would prevent any oxygen-driven oxidation. However, this would require some more sophisticated equipment.

In summary, demonstration of a well-performed distillation requires the use of a modern distillation system, and a thermometer is a must! High temperature tolerant compounds are preferable, for example a solution of common table salt (NaCl) in water, since table salt is very stable at high temperatures. In addition, it is recommended to constantly stir the mixture during the distillation process, in order to allow for adequate dispersion of the heat (and prevent any material at the bottom of the flask from overheating and breaking down, much like the rice at the bottom of the pot).

Dr. Avi Saig
Department of Neurobiology and Davidson Institute of Science Education
Weizmann Institute of Science

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